A few things to consider. The newer 3Ds have a slightly different tread pattern, and possibly tread composition, compared to the previous-generation Winter Sport M3s. Ideally all four winter tires should be the same make/model. But the difference is probably small enough that it won't matter.

Keep in mind that your front unused M3s might not have a ton of life left on them. They have zero miles, but tires do break down over time. They're already four years old, and I believe that 6 years or more is the period where you should consider replacing tires due to rubber decay. My numbers might be off here, so others should chime in. But don't be surprised if you're looking at replacing them in a couple of years.

Of the two choices, 265 vs. 285, I'd probably do the 285s. Ideally winter tires should be narrower than summers to help dig down into the snow and get traction, as opposed to floating on top like pontoons. But 285 over 265 probably isn't that significant. 215 or 225 vs. 285 would be significant, but who's going to put 215s on their M3? I would be more concerned about stretching a 265mm tire over that wide 10.5" wheel. 285 would be more appropriate.

And as far as increasing the stagger to 40mm goes, I don't think that will be too much of an issue, either. An increased stagger might present a handling problem in the setting of summer tires in dry, warm conditions when pushing the car hard, but this clearly will not be the case during a Toronto winter.

It might be worth it to do some research on TireRack's website regarding the appropriateness of the Dunlops for a Toronto winter. If this will be your only winter vehicle, you might want to consider a tire with a bit more snow and ice grip, but still "performance winter," such as the Blizzak LM60. Then you can probably also have more leeway with sizing and maintaining proper stagger. Or use your OEM wheels for winter and save the TE37s for summer duty. That way you could also do a narrower 255 or 265 in back.